I thought of what the game should be about. A villain ( Haven't thought about it.) kidnaps the 7 Koopalings including Bowser Jr. Bowser then has to save them. You can use him to shoot fire, jump and punch. You rescue one Koopaling each time you defeat a boss.

The bosses could be the mushroom people or any recruits Mario could get. Heck, Mario could be a boss a few times that escapes when defeated in all but the last level. As for the order in which the Koopa kids are rescued, I don't think the order should matter.